An inertial navigation system for an aircraft, may utilize three accelerometers for measuring accelerations in three perpendicular directions, and with the first and second integrals of each accelerometer indicating the speed and position of the aircraft. Three gyroscopes may also be provided to indicate the precise direction in which each accelerometer is pointed, or in some systems may even turn the table on which the accelerometers are mounted to maintain a precise orientation. The gyroscopes are among the least accurate components of the system, and therefore the assumed and actual orientation of each accelerometer may be different. Large errors in computing the position of the aircraft can arise when an accelerometer direction which is assumed to be horizontal (with respect to the local horizontal direction) is not precisely horizontal. In that case, the effect of gravity is to introduce an erroneous acceleration into the system. For example, if an aircraft is flying at a constant forward speed so there is no forward acceleration, but the accelerometer which measures forward acceleration and which is supposed to be held horizontal is actually tilted by 0.01.degree., then the accelerometer will register a constant acceleration of G sin 0.01.degree. , where G is the acceleration of gravity (32 feet per second.sup.2). After one hour, this "ghost" acceleration would result in an error of 14 miles per hour in aircraft velocity, and a position error of 7 miles. The error would grow with time so that after three hours, the velocity error would be 42 mph and the position error would be 63 miles. It may be noted that, due to the earth's curvature, a Shuler oscillation phenomenon occurs which affects the amount of error in this situation, but the error is still of great importance.
Various techniques have been utilized to minimize position and/or velocity errors caused by deviation of the inertial horizontal or vertical from the actual local horizontal or vertical. Where the aircraft is flying over land and the locations of landmarks are known along the route, then precise corrections can be made. When landmarks are not available over land, corrections can be made by measuring the true velocity of the aircraft relative to the ground, and comparing this to the assumed velocity in the inertial navigation system. The true velocity can be determined by transmitting radar signals towards the ground and detecting the Doppler shift of the reflected radar waves. Correcting errors in the assumed velocity of the inertial system is useful, but does not provide as great an accuracy as where the precise position or orientation of the aircraft can be determined.
When the aircraft is flying over water, even velocity measurements cannot be made with great accuracy, because of ocean currents. That is, the velocity of the aircraft relative to a location on the ocean surface can be precisely determined, but that ocean surface may be moving at a speed such as a few miles an hour because it is in a current. Thus, the lack of the possibility of updating an inertial system by use of landmarks or even by accurate velocity measurements, has been an important limitation on the accuracy of inertial navigation systems when used in aircraft flying over large areas of the oceans. A system which enabled corrections to be made to the inertial reference, particularly as to its position, while it is being flown over large expanses of water, would be of great value in increasing the accuracy of inertial navigation systems.